Many cell types in vertebrates express proteins which are capable of detecting the presence of a diverse array of pathogens, including bacteria, mycoplasmas, and fungi, a capacity which is known as innate immunity. Upon recognition of invading pathogens, these cells produce specific inflammatory cytokines which signal the immune system to mount a response. A small class of proteins called toll-like receptors (TLRs) has recently been shown to play a central role in transducing the signal from invading pathogens across the plasma membrane. The mechanism of pathogen recognition is not presently understood from a structural perspective. The objective of the research propsed here is to characterize the interaction between TLRs and various ligands using biophysical and structural approaches according to the following specific aims: (1) overexpression and purification of the extracellular domains of human TLR2 or TLR4 along with the adaptor proteins CD14 and MD-2; (2) biophysical characterization of the binding affinities of various ligands and adaptor proteins using analytical ultracentrifugation, fluorescence anisotropy, and surface plasmon resonance; (3) determination of the atomic structure of the extracellular domains of TLR2 or TLR4 using x-ray crystallography.